Composition and method

ABSTRACT

A composition includes a sintered mass having a plurality of cores and a grain boundary layer disposed between each of the plurality of cores. The core includes a transition metal oxide, and grain boundary layer includes a sintering additive, a grain growth inhibitor additive and a grain boundary additive.

RELATED APPLICATIONS

This application is a non-provisional application that claims priority to provisional U.S. Pat. application Ser. No. 60/991,871, filed Dec. 3, 2007; the disclosure of which is hereby incorporated by reference.

BACKGROUND

1. Technical Field

The invention includes embodiments that relate to a composition for use as a surge protector and/or varistor. The invention includes embodiments that relate to a method of using the composition, or derived device.

2. Discussion of Art

A varistor is an electronic component with a non-ohmic current-voltage characteristic. Varistors may protect circuits against excessive transient voltages by incorporating them into the circuit in such a way that, when triggered, they will shunt the current created by the high voltage away from the sensitive components. A varistor may be known as Voltage Dependent Resistor or VDR.

A type of varistor is the Metal Oxide Varistor (MOV). This contains a ceramic mass of zinc oxide grains, in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbour forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. When a small or moderate voltage is applied across the electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. When a large voltage is applied, the diode junctions break down because of the avalanche effect, and a large current flows. The result of this behaviour is a highly nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages.

A varistor remains non-conductive as a shunt mode device during normal operation when voltage remains well below its “clamping voltage”. If a transient pulse (often measured in joules) is too high, the device may melt, burn, vaporize, or otherwise be damaged or destroyed. This unacceptable (catastrophic) failure occurs when “Absolute Maximum Ratings” are exceeded. Varistor degradation is defined using curves that relate current, time, and number of transient pulses. A varistor fully degrades when its “clamping voltage” has changed by 10 percent. A fully-degraded varistor may remain functional, having no catastrophic failure, and may not be visually damaged.

It may be desirable to have a composition or article with properties and characteristics that differ from those properties of currently available compositions and articles.

BRIEF DESCRIPTION

In one embodiment, a composition is provided that includes a sintered mass having a plurality of cores and a grain boundary layer disposed between each of the plurality of cores. The core includes a transition metal oxide, and grain boundary layer includes a sintering additive and a grain growth inhibitor additive.

In one embodiment, a composition is provided that includes a sintered reaction product of transition metal oxide particles that may have an average diameter less than about 1 micrometer; and sintering additive particles that may have an average diameter less than about 1 micrometer; and the grain growth inhibitor additive particles may have an average diameter less than about 1 micrometer.

In one embodiment, a composition is provided. The composition includes a sintered mass of particles comprising a transition metal oxide, a sintering additive, and a grain growth inhibitor additive. The sintered mass may have a density greater than about 98 percent of theoretical density for a composition comprising the transition metal oxide.

In one embodiment, a composition includes sintered particles that include a transition metal oxide, a sintering additive, a grain growth inhibitor additive and defining grains. The grains may have grain boundaries that define the grains to have an average grain size of less than about 0.8 micrometers.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the electric field versus the current density (current voltage graph) for the composition in accordance with one embodiment of the invention and the comparative sample.

FIG. 2 shows the SEM micrographs of the composition of the control blank.

FIG. 3 shows the SEM micrographs of the composition in accordance with one embodiment of the invention.

DETAILED DESCRIPTION

The invention includes embodiments that relate to a composition for use as a surge protector and/or varistor. The invention includes embodiments that relate to a method of using the composition, or the derived device.

As used herein, the term sintering is a method for making objects from particles or powder, by heating the material (below its melting point) until its particles adhere to each other. Sintered refers to particles or powder that has undergone a sintering process. A sintered mass refers to the formed shape that is the result of the sintering of powders or particulate. In the sintered mass, formerly discrete particles or powder grains retain a core, and the interstitial area from one core to another core is at least partially filled with a grain boundary layer that separates the cores.

In one embodiment, a composition includes a sintered mass. The sintered mass includes a plurality of particle cores and a grain boundary layer disposed between each of the plurality of particle cores. Each of the cores may include a transition metal oxide. The grain boundary layer includes a sintering additive, a grain boundary additive, and/or a breakdown voltage additive.

In one embodiment, the particle core may include a transition metal. In one embodiment, the transition metal may be a transition metal oxide. Examples of transition metal oxides include but are not limited to zinc oxide, tin oxide, and titanium oxide. In one embodiment, the transition metal oxide includes a zinc oxide. The amount of the transition metal oxide, by weight, may be greater than about 80 percent based on the total weight of the sintered mass. In one embodiment, the amount may be in a range of from about 80 weight percent to about 85 weight percent, from about 85 weight percent to about 90 weight percent, or from about 90 weight percent to about 95 weight percent or from about 95 weight percent to about 98 weight percent based on the total weight of the sintered mass.

In one embodiment, the grain boundary layer is disposed between each of the plurality of the cores. The grain boundary layer includes a sintering additive. In one embodiment, the sintering additive may include one or more of aluminum, lithium, antimony, bismuth, cobalt, chromium, manganese, nickel, magnesium, or silicon. In one embodiment, the sintering additive may include a combination of two or more of the foregoing. In one embodiment, the sintering additive includes one or more of SiO₂, Mn₂O₃, NiO, MnO₂, MnCO₃, Li₂CO₃, or LiBiO₃. In one embodiment, the sintering additive may include one or more of Li₂CO₃, or LiBiO₃. In one embodiment, the sintering additive may include a combination of two or more of the foregoing. The selection of the sintering additive may be based on one or more factors as the sintering additives differ in efficacy and effect. Such factors may include the desired sintering temperature, the sintering pressure, the material performance, and the desired grain characteristics.

The sintering additive may be present in an amount that is less than about 15 percent by weight, based on the total weight of the sintered mass. In one embodiment, the sintering additive amount may be in a range of from about 15 percent to about 12 percent, from about 12 percent to about 10 percent, from about 10 percent to about 8 percent, from about 8 percent to about 4 percent, from about 4 percent to about 2 percent, from about 2 percent to about 0.5 percent, from about 0.5 percent to about 0.3 percent, or from about 0.3 percent to about 0.1 percent, or from about 0.1 percent to about 0.03 percent.

In one embodiment, the grain boundary includes a grain growth inhibitor additive. In one embodiment, the grain growth inhibitor additive may include one or more of Sb₂O₃, CaO, Al₂O₃, MgO, or Fe₂O₃. In one embodiment, the grain growth inhibitor may consist essentially of only one of the foregoing. The selection of the grain growth inhibitor additive may be based on one or more factors as the grain growth inhibitor additive differ in efficacy and effect. Such factors may include the desired sintering temperature, the sintering pressure, the material performance, and the desired grain characteristics. In one embodiment, the grain growth inhibitor additive may inhibit grain growth to maintain relatively smaller grains. The grain growth inhibitor additive may control the grain size distribution, as well. In one embodiment, the grain growth inhibitor additive may be present in an amount in a range of from about 0.1 weight percent to about 0.5 weight percent, from about 0.5 weight percent to about 1.5 weight percent, or from about 1.5 weight percent to about 3 weight percent.

In one embodiment, the grain growth inhibitor additive may include a combination of two or more of the foregoing. In one embodiment, the grain growth inhibitor additive may be present in the sintered mass in an amount, by weight, that is less than about 10 percent based on the total weight of the sintered mass. In one embodiment, the grain growth inhibitor additive amount may be in a range of from about 10 weight percent to about 8 weight percent, from about 8 weight percent to about 6 weight percent, 6 weight percent to about 4 weight percent, from about 4 weight percent to about 2 weight percent, from about 2 weight percent to about 1 weight percent, from about 1 weight percent to about 0.5 weight percent, from about 0.5 weight percent to about 0.1 weight percent, or less than about 0.1 weight percent.

In one embodiment, the composition may further include a grain boundary additive. In one embodiment, the grain boundary additive includes a breakdown voltage additive. In one embodiment, the grain boundary additive may enhance the grain boundary barrier. In one embodiment, the grain boundary additive may include one or more of CO₃O₄, CO₂O₃, Cr₂O₃, Bi₂O₃, Pr₂O₃, NiO, or SnO₂. In one embodiment, the grain boundary additive consists essentially of only one of the foregoing. The selection of the grain boundary additive may be based on one or more factors as the grain boundary additive differ in efficacy and effect. Such factors may include the desired sintering temperature, the sintering pressure, the material performance, and the desired grain characteristics. The grain boundary additive may be present in an amount less than about 1 weight percent. In one embodiment, the grain boundary additive may be present in an amount in a range of from about 0.01 weight percent to about 0.5 weight percent, from about 0.5 weight percent to about 0.75 weight percent, or from about 0.75 weight percent to about 1 weight percent. In one embodiment, the composition is free of CO₂O₃. In another embodiment, the amount of CO₂O₃ is less than about 0.05 weight percent.

In one embodiment, the additive may include a combination of two or more of the foregoing. In one embodiment, the grain boundary additive may be present in the sintered mass in an amount, by weight, that is less than about 10 percent based on the total weight of the sintered mass. In one embodiment, the grain boundary additive is present in an amount in a range of from about 10 weight percent to about 8 weight percent, from about 8 weight percent to about 6 weight percent, from about 6 weight percent to about 4 weight percent, from about 4 weight percent to about 2 weight percent, from about 2 weight percent to about 1 weight percent, from about 1 weight percent to about 0.5 weight percent, from about 0.5 weight percent to about 0.1 weight percent, or less than about 0.1 weight percent.

In one embodiment, the average distance from one core to an adjacent core in the plurality of cores is less than about 1 micrometer. In one embodiment, the average distance may be in a range of from about 1 micrometer to about 0.8 micrometers, or from about 0.8 micrometers to about 0.5 micrometers. In another embodiment, the average distance may be in a range of from about 500 nanometers to about 400 nanometers, from about 400 nanometers to about 300 nanometers, from about 300 nanometers to about 250 nanometers, from about 250 nanometers to about 200 nanometers, from about 200 nanometers to about 150 nanometers, from about 150 nanometers to about 100 nanometers, from about 100 nanometers to about 50 nanometers, or less than about 50 nanometers.

In one embodiment, the average diameter of the core in the plurality of cores is less than about 1 micrometer. In one embodiment, the average diameter may be in a range of from about 1 micrometer to about 0.8 micrometers, or from about 0.8 micrometers to about 0.5 micrometers. In another embodiment, the average diameter may be in a range of from about 500 nanometers to about 400 nanometers, from about 400 nanometers to about 300 nanometers, from about 300 nanometers to about 250 nanometers, from about 250 nanometers to about 200 nanometers, from about 200 nanometers to about 150 nanometers, from about 150 nanometers to about 100 nanometers, from about 100 nanometers to about 50 nanometers, or less than about 50 nanometers.

The micro-structure or nano-structure of the composition may be expressed in terms of an average distance from one core to an adjacent core in the sintered mass. The average distance from one core to an adjacent core in the sintered mass may be less than 5 micrometers. In one embodiment, the average distance may be in a range of from about 1 micrometer to about 0.8 micrometers, or from about 0.8 micrometers to about 0.5 micrometers. In another embodiment, the average distance may be in a range of from about 500 nanometers to about 400 nanometers, from about 400 nanometers to about 300 nanometers, from about 300 nanometers to about 250 nanometers, from about 250 nanometers to about 200 nanometers, from about 200 nanometers to about 150 nanometers, from about 150 nanometers to about 100 nanometers, from about 100 nanometers to about 50 nanometers, or less than about 50 nanometers. An exemplary core-to-core average distance may be in a range of from about 35 nanometers to about 75 nanometers.

The distance of one core to another core, coupled with the core size, may affect the average thickness of the grain boundary layer. In one embodiment, the average thickness of the grain boundary layer may be less than about 1 micrometer. In another embodiment, the average thickness may be in a range of from about 1 micrometer to about 0.8 micrometers, or from about 0.8 micrometers to about 0.5 micrometers. In yet another embodiment, the average thickness may be in a range of from about 500 nanometers to about 400 nanometers, from about 400 nanometers to about 300 nanometers, from about 300 nanometers to about 250 nanometers, from about 250 nanometers to about 100 nanometers, from about 100 nanometers to about 50 nanometers, from about 50 nanometers to about 35 nanometers, from about 35 nanometers to about 20 nanometers, or less than about 20 nanometers.

The grain boundary layer thickness, may be expressed as a mean value in nanometers. The mean value for the grain boundary layer may be less than about 50 nanometers. In one embodiment, the mean value may be in a range of from about 50 nanometers to about 10 nanometers, from about 10 nanometers to about 1 nanometer, or from about 1 nanometer to about 0.1 nanometers.

In addition to such factors as the uniformity of core diameters, the uniformity of distribution of materials, and the uniformity of the grain boundary layer, the average distance of the cores from one to another may affect the performance, properties and characteristics of the varistor device made therefrom. Particularly, the diode junction performance, and the number of diode junctions per unit volume, may flow directly from the core spacing parameter.

In one embodiment, the sintered mass may have a dielectric strength or breakdown field of greater than about 0.5 kV/mm. In one embodiment, the dielectric strength or breakdown field is in a range of from about 0.5 kV/mm to about 1 kV/mm, from about 1 kV/mm to about 1.5 kV/mm, from about 1.5 kV/mm to about 2 kV/mm, from about 2 kV/mm to about 2.5 kV/mm, from about 2.5 kV/mm to about 2.8 kV/mm, or greater than about 2.8 kV/mm. In one embodiment, the sintered mass may have a non-linearity coefficient (α) of greater than 25. In one embodiment, the non-linearity coefficient (α) may be in a range of from about 25 to about 50, from about 50 to about 75, from about 75 to about 100, from about 100 to about 125, from about 125 to about 140, or greater than about 140.

The thermal profile may play a role in the melt temperature of the electrode of the MOV device. If the thermal profile is higher than the electrode melt temperature, then the electrode may be melted, damaged or destroyed. A higher thermal excursion during manufacture or sinter may then require an electrode with a corresponding melt temperature suitable for use after exposure to that temperature. Lower temperature capable electrode materials may be economically desirable, if the other performance parameters are correct. In addition, if the thermal profile shows a temperature excursion too high, the micro-structure or nano-structure may change and the sintered particles may melt and flow together rather than remain as a sintered mass. This may need to be balanced, as at least some heat is needed to get the particles to sinter in the first instance.

In one embodiment, a sintered mass may be produced by mixing a transition metal oxide, a sintering additive, and a grain boundary additive under defined conditions to form a mixture. The mixture can be treated to a determined temperature profile. In one embodiment, the temperature profile includes exposure to a sinter temperature of less than about 1050 degrees Celsius. The composition may have a thermal profile also known as thermal history that may include exposure to a sintering temperature of not greater than about 1050 degrees Celsius. In one embodiment, the thermal profile includes exposure to a sinter temperature in a range of from about 1050 degrees Celsius to about 1000 degrees Celsius, from about 1000 degrees Celsius to about 950 degrees Celsius, from about 950 degrees Celsius to about 900 degrees Celsius, from about 900 degrees Celsius to about 875 degrees Celsius, or from about 875 degrees Celsius to about 850 degrees Celsius.

In one embodiment, the composition includes a sintered reaction product of transition metal oxide particles that have an average diameter that is less than about 1 micrometer; and sintering additive particles having an average diameter that is less than about 1 micrometer. The grain growth inhibitor additive particles may have an average diameter that is less than about 1 micrometer. Due to the change in available surface area, and packing tendencies, particles of different sizes may form sintered masses having differing properties and characteristics.

In one embodiment, the composition includes a sintered mass of particles that may include a transition metal oxide, a sintering additive, and a grain growth inhibitor additive. The sintered mass may have a density that is greater than 98 percent of theoretical density for a composition comprising the transition metal oxide.

In one embodiment, the composition includes sintered particles that include a transition metal oxide, a sintering additive, and a grain growth inhibitor additive and defining grains. The grains may have grain boundaries that define the grains to have an average grain size of less than about 0.8 micrometers. In one embodiment, the method includes contacting a transition metal oxide, a sintering additive, and a grain growth inhibitor additive to form a mixture. The mixture may be treated to a temperature profile. In one embodiment, the temperature profile includes exposure to a sinter temperature of less than about 1050 degrees Celsius.

EXAMPLES

The following examples illustrate methods and embodiments in accordance with the invention, and as such do not limit the claims. Unless specified otherwise, all ingredients may be commercially available from such common chemical suppliers as Alpha Aesar, Inc. (Ward Hill, Mass.), Sigma Aldrich (St. Louis, Mo.), Spectrum Chemical Mfg. Corp. (Gardena, Calif.), and the like.

Examples 1 through 3 are prepared by mixing, calcining, ball milling, and sintering. The sintering is performed in a Uniaxial Press to make a puck for each sample that is about 1 inch in diameter. The various components and the weight percent for each of the components for examples 1 to 3 are given in Table 1.

Example 1

A mixture is formed from zinc oxide, and additives selected from cobalt, antimony, nickel, and chromium oxide nanopowders with bismuth, silicon, manganese oxide nanopowders in a ratio given in Table 1. The zinc oxide is commercially obtainable from Horsehead Coporation, (Monaca, Pa.). The additives are commercially obtainable from Nanostructured and Amorphous Materials Inc. (Houston, Tex.).

TABLE 1 Composition (Weight Sample Sample Sample Comparative Comparative percent) 1 2 3 Sample 1 Sample 2 ZnO 94 85.5 94.69 83.39 92.21 Bi₂O₃ 0.5 2 3 2.12 1.40 Sb₂O₃ 1 3 1.5 6.34 3.75 Al₂O₃ — 2 0.01 0.04 — SiO₂ 2 3 — 0.43 0.07 Cr₂O₃ 0.5 — — — 1.02 MnO — — — 0.9 0.4 Mn₂O₃ 0.5 — 0.1 — — MgO — 2 — — — Fe₂O₃ — — — 0.01 0.04 Co₂O₃ — — — 1.13 1.17 Co₃O₄ 0.5 2.5 0.5 — — NiO 1 — 0.2 1.27 — SnO₂ — — — — 0.93

The materials form a mixture in a mixed oxide wet process. The mixture is milled in a ball mill for about 6 hours in a ratio materials:ball:isopropyl alcohol=1:5:2 to form a slurry. The slurry is dried at 100 degrees Celsius. The dried powder is sieved and calcined at 550 degrees Celsius for about 2 hours in a Thermolyne 1400 furnace. The calcined powder is then ball milled for about 4 hours. The slurry formed is dried at 100 degrees Celsius and the dried powder is sieved. The powder is then pressed into pellets (thickness of about 1.5 millimeters) with a force of about 10000 pounds for about 1 minute. The pellet is sintered at temperatures from about 1000 degrees Celsius and 1050 degrees Celsius. The sintering is done in two different profiles including one and two steps in a Uniaxial Press for about 2 hours. The first profile is carried out at about at 1050 degrees Celsius at a heating rate of about 5 degrees Celsius per minute for about 2 hours and is allowed to cool. The second profile is carried out at about at 1000 degrees Celsius at a rate of about 10 degrees Celsius per minute for about 0.1 hours. Following this a second step sintering at a temperature of about 925 degrees Celsius to 975 degrees Celsius at a heating rate of about 10 degrees Celsius per minute is carried out for about 2 hours. The resultant product is Sample 1, which has the compositional distribution as indicated in Table 1.

Example 2

A mixture is formed from zinc oxide, and additives selected from oxide nanopowders cobalt, and antimony, with nanopowder oxides of bismuth, silicon, aluminum and magnesium in a ratio given in Table 1.

The materials are mixed using a mixed Oxide Wet Process. The mixture is milled in a ball mill for about 6 hours in a ratio materials:ball:isopropyl alcohol=1:5:2 to form a slurry. The slurry is dried at 100 degrees Celsius. The dried powder is sieved and calcined at 550 degrees Celsius for about 2 hours in a Thermolyne 1400 furnace. The calcined powder is then ball milled for about 4 hours. The slurry formed is dried at 100 degrees Celsius and the dried powder is sieved. The powder is then pressed into pellets (thickness of about 1.5 millimeters) with a force of about 10000 pounds for about 1 minute. The pellet is sintered in a Uniaxial Press at different temperatures for about 2 hours at about 950 degrees Celsius, about 1050 degrees Celsius at a rate of about 5 degrees Celsius per minute. The resultant product is Sample 2, which has the compositional distribution indicated in Table 1.

Example 3

A mixture is formed from zinc oxide (from Horsehead Coporation, Monaca, Pa.), and additives selected from powders of cobalt, nickel, and antimony-based materials (from Nanostructured and Amorphous Materials Inc, Houston, Tex.), and with powders of bismuth, aluminum and manganese-based materials in amounts as given in Table 1.

The materials are mixed using a mixed oxide wet process. The mixture is milled in a ball mill for about 6 hours in a ratio of powder materials:ball:isopropyl alcohol=1:5:2 to form a slurry. The slurry is dried at 100 degrees Celsius. The dried powder is sieved and calcined at 550 degrees Celsius for about 2 hours in a Thermolyne 1400 furnace. The calcined powder is then ball milled for about 4 hours. The slurry formed is dried at 100 degrees Celsius and the dried powder is sieved. The powder is then pressed into pellets (thickness of about 1.5 millimeters) with a force of about 10000 pounds for about 1 minute. The pellet is sintered in a pressureless mode at different temperatures for about 2 hours at about 850 degrees Celsius, about 900 degrees Celsius, about 950 degrees Celsius, about 1000 degrees Celsius and about 1050 degrees Celsius at a rate of about 5 degrees Celsius per minute. The resultant product is Sample 3, which has the compositional distribution indicated in Table 1.

Current-Voltage (I-V) Measurement:

A 10 kiloOhm or 100 MegaOhm resistor is connected in parallel to the varistor and a voltage is applied. V1, the total voltage on sample and varistor is measured using a high voltage probe. V2, the voltage on the resistor is measured by a multimeter. V2 is used to calculate the current flowing through the varistor. V1-V2 is the voltage on the Samples 1-6. To measure I-V curve, at low voltage, a 100 MOhm resistor is used until the voltage on it is higher than about 100 Volts. A 10 kilo Ohm resistor is used to measure the I-V curve under high voltage (higher than about 100 Volts).

FIG. 1 shows the results of metal oxide varistor materials of Samples 1-3 relative to a commercially available metal oxide varistor material. The materials of Samples 1-3 display relatively better breakdown strength and relatively better nonlinearity compared to Comparative Sample 1. The breakdown fields (electric fields when current density is 1 milliAmp per square centimeter) and nonlinearity coefficient α calculated are summarized in Table 2.

Table 2 shows that the metal oxide varistor materials of Samples 1-3 perform better after low temperature firing. For example, Sample 3 gives a breakdown field of greater than about 1700 volts per millimeter and a good nonlinearity coefficient (α) of about 77, but still has a low sintering temperature of 850 degrees Celsius.

TABLE 2 Breakdown fields and nonlinearity of metal oxide varistor materials of Samples 1-3 and commercially available metal oxide varistor material Sintering Breakdown Temperature (° C.)/ Field Nonlinearity Composition Time (Hours) (V/mm) coefficient α Sample 1 1000/2 1343 63 1050/2 972 138 Sample 2  950/2 2800 40 1000/2 2216 18 Sample 3  850/2 1710 77  900/2 546 19  950/2 515 77 1000/2 400 42 1050/2 315 79 Comparative NA 125 22 Sample 1

Microstructure formation may depend at least in part on the sintering profile. Grain size is found to increase at higher sintering temperature. FIGS. 2-3 compare the microstructure of a commercially available metal oxide varistor material Comparative Sample 1 (FIG. 2) with a metal oxide varistor material of Example 3 (FIG. 3). The average grain size of the metal oxide varistor materials of Sample 3 (sintered at 850 degrees Celsius) is less than 1 micrometer; and, this is in comparison to the commercially available metal oxide varistor material Comparative Sample 1 that has a grain size that is greater than 10 micrometers. Several phases may coexist in the metal oxide varistor materials of Samples 1-3. These phases may include the major conductive phase of less than 1 micrometer in size and one or more secondary phases located at the grain boundaries and in the grain boundary layer, which itself may include various dopants and sintering additives.

Example 4

A mixture is formed from zinc oxide, and additives selected from oxide powders cobalt, and antimony, with powder oxides of bismuth, silicon, aluminum and chromium in amounts as given in Table 3. Unless otherwise indicated, the powders are nanoscale and have a narrow size distribution.

The materials are mixed using a mixed Oxide Wet Process. The mixture is milled in a ball mill for about 6 hours in a ratio of ingredients:ball:isopropyl alcohol of 1:5:2 to form a slurry. The slurry is dried at 100 degrees Celsius. The dried powder is sieved and calcined at 550 degrees Celsius for about 2 hours in a THERMOLYNE 1400 furnace. The calcined powder is then ball milled for about 4 hours. The slurry formed is dried at 100 degrees Celsius and the dried powder is sieved. The powder is then pressed into pellets (thickness of about 1.5 millimeters) with a force of about 10000 pounds for about 1 minute. The pellet is sintered in a Uniaxial Press at different temperatures for about 2 hours at about 950 degrees Celsius, about 1050 degrees Celsius at a rate of about 5 degrees Celsius per minute. The resultant product is Sample 4, which has the compositional distribution indicated in Table 3.

Example 5

A plurality of mixtures are formed from zinc oxide, and additives selected from cobalt, antimony, nickel, and chromium-based powders with bismuth, silicon, manganese-based powders, each in an amount as given in Table 3. The powders, unless context or language indicates otherwise, are nano-scale and have an average diameter that is less than 100 nanometers, and a relatively narrow and mono-modal size distribution.

The materials form a mixture in a mixed oxide wet process. The mixture is milled in a ball mill for about 6 hours in a ratio ingredients:ball:isopropyl alcohol of 1:5:2 to form a slurry. The slurry is dried at 100 degrees Celsius. The dried powder is sieved and calcined at 550 degrees Celsius for about 2 hours in a Thermolyne 1400 furnace. The calcined powder is then ball milled for about 4 hours. The slurry formed is dried at 100 degrees Celsius and the dried powder is sieved. The powder is then pressed into pellets (thickness of about 1.5 millimeters) with a force of about 10,000 pounds for about 1 minute. The pellet is sintered at temperatures from about 1000 degrees Celsius and 1050 degrees Celsius. The sintering is done in two different profiles including one and two steps in a Uniaxial Press for about 2 hours. The first profile is carried out at about at 1050 degrees Celsius at a heating rate of about 5 degrees Celsius per minute for about 2 hours and is allowed to cool. The second profile is carried out at about at 1000 degrees Celsius at a rate of about 10 degrees Celsius per minute for about 0.1 hours. Following, a second step includes sintering at a temperature of about 925 degrees Celsius to 975 degrees Celsius. The sintering is at a heating ramp up rate of about 10 degrees Celsius per minute for about 2 hours. The resultant product is Sample 5, which has the compositional distribution as indicated in Table 3.

Example 6

A mixture is formed from zinc oxide, and additives selected from powders of cobalt, lithium, nickel, and antimony-based materials, with powders of bismuth, and aluminum-based materials in amounts as given in Table 3.

The materials are mixed using a mixed oxide wet process. The mixture is milled in a ball mill for about 6 hours in a ratio of powder ingredients:ball:isopropyl alcohol of 1:5:2 to form a slurry. The slurry is dried at 100 degrees Celsius. The dried powder is sieved and calcined at 550 degrees Celsius for about 2 hours in a THERMOLYNE 1400 furnace. The calcined powder is then ball milled for about 4 hours. The slurry formed is dried at 100 degrees Celsius and the dried powder is sieved. The powder is then pressed into a plurality of pellets (thickness of about 1.5 millimeters) with a force of about 10000 pounds for about 1 minute.

The pellets are sintered in a pressureless mode at different temperatures for about 2 hours. The temperatures are: about 800 degrees Celsius (sample 6), about 850 degrees Celsius (sample 7), about 900 degrees Celsius (sample 8), about 950 degrees Celsius (sample 9), about 1000 degrees Celsius (sample 10), and about 1050 degrees Celsius (sample 11), each at a rate of about 5 degrees Celsius per minute. Sample 6 is subsequently subjected to each of the other temperature profiles. The resultant product pellets are represented in Samples 6-11, which have the compositional distribution indicated in Table 3. Additional samples 12 et seq. have the compositional makeup as indicated in Table 3, and are subject to the temperature profile of Sample 7 (850 degrees Celsius) and are prepared in the same manner as the rest of the Samples in the this example.

Table 3 shows that the metal oxide varistor materials may perform relatively well, displaying a breakdown field of greater than about 1700 volts per millimeter and a good nonlinearity coefficient (α) of greater than about 75, but still having a relatively low sintering temperature.

TABLE 3 Composition Samples Sample Sample Sample (Wt percent) Sample 4 Sample 5 Sample 6 7-11 12 13 14 ZnO 85.5 94 94.69 94.69 95.0 84.0 94.0 Bi₂O₃ 2 0.5 3 3 3.5 3 3 Sb₂O₃ 3 1.4 1.5 1.5 0.2 3 0.1 Al₂O₃ 3 — 0.01 0.01 0.1 — — SiO₂ 3 2 — — 0.1 0.5 1.0 Cr₂O₃ 0.95 0.04 — — 0.1 — — MnO — — — — 0.1 0.5 — Mn₂O₃ — 0.6 — — 0.1 — — MgO 0.05 — — — 0.1 1 — Fe₂O₃ — — — — 0.1 0.5 — Co₂O₃ — — — — 0.1 2.5 — Co₃O₄ 2.5 0.5 0.5 0.5 0.1 — — NiO — 0.96 0.2 0.2 0.1 3 — SnO₂ — — — — 0.1 — — Li₂CO₃ — — 0.1 0.1 0.1 1.5 0.9 LiBiO₃ — — — — 0.1 — 1 CaO — — — — 0.1 0.5 — Breakdown Field >1800 >1800 >1850 — — — — (V/mm) Nonlinearity >75 >80 >75 — — — — coefficient (α)

In the specification and claims, reference will be made to a number of terms have the following meanings. The singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term such as “about” is not to be limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Similarly, “free” may be used in combination with a term, and may include an insubstantial number, or trace amounts, while still being considered free of the modified term.

As used herein, the terms “may” and “may be” indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of “may” and “may be” indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances the modified term may sometimes not be appropriate, capable, or suitable. For example, in some circumstances an event or capacity can be expected, while in other circumstances the event or capacity cannot occur—this distinction is captured by the terms “may” and “may be”.

Reactants and components referred to by chemical name or formula in the specification or claims hereof, whether referred to in the singular or plural, may be identified as they exist prior to coming into contact with another substance referred to by chemical name or chemical type (e.g., another reactant or a solvent). Preliminary and/or transitional chemical changes, transformations, or reactions, if any, that take place in the resulting mixture, solution, or reaction medium may be identified as intermediate species, master batches, and the like, and may have utility distinct from the utility of the reaction product or final material. Other subsequent changes, transformations, or reactions may result from bringing the specified reactants and/or components together under the conditions called for pursuant to this disclosure. In these, other subsequent changes, transformations, or reactions the reactants, ingredients, or the components to be brought together may identify or indicate the reaction product or final material.

The embodiments described herein are examples of articles, compositions, and methods having elements corresponding to the elements of the invention recited in the claims. This written description may enable those of ordinary skill in the art to make and use embodiments having alternative elements that likewise correspond to the elements of the invention recited in the claims. The scope of the invention thus includes articles, compositions and methods that do not differ from the literal language of the claims, and further includes other articles, compositions and methods with insubstantial differences from the literal language of the claims. While only certain features and embodiments have been illustrated and described herein, many modifications and changes may occur to one of ordinary skill in the relevant art. The appended claims cover all such modifications and changes. 

1. A composition, comprising: a sintered mass having a plurality of cores; a grain boundary layer disposed between each of the plurality of cores; and wherein the core comprises a transition metal oxide, and the grain boundary layer comprises a sintering additive and a grain growth inhibitor additive.
 2. The composition as defined in claim 1, wherein the transition metal oxide is zinc oxide.
 3. The composition as defined in claim 1, wherein the sintering additive comprises one or more material comprising lithium, antimony, bismuth, cobalt, manganese, or silicon.
 4. The composition as defined in claim 3, wherein the sintering additive is LiBiO₃, Li₂CO₃, Mn₂O₃, MnO₂, MnCO₃, Sb₂O₅, or SiO₂.
 5. The composition as defined in claim 1, wherein the amount of the transition metal oxide is greater than about 80 percent by weight, based on the total weight of the sintered mass.
 6. The composition as defined in claim 1, wherein the amount of the sintering additive is less than about 15 percent by weight, based on the total weight of the sintered mass.
 7. The composition as defined in claim 1, wherein the grain growth inhibitor additive comprises one or more of SiO₂, Sb₂O₃, CaO, Al₂O₃, MgO, or Fe₂O₃.
 8. The composition as defined in claim 1, wherein the amount of the grain growth inhibitor additive is less than about 10 percent by weight, based on the total weight of the sintered mass.
 9. The composition as defined in claim 1, further comprising a grain boundary additive.
 10. The composition as defined in claim 8, wherein the grain boundary additive comprises CO₃O₄, Cr₂O₃, Bi₂O₃, Pr₂O₃, NiO, or SnO₂.
 11. The composition as defined in claim 8, wherein the amount of the grain boundary additive is less than about 10 percent by weight, based on the total weight of the sintered mass.
 12. The composition as defined in claim 1, wherein the composition is substantially free of CO₂O₃.
 13. The composition as defined in claim 1, wherein an average distance from one core to an adjacent core in the plurality of cores is less than about 1 micrometer.
 14. The composition as defined in claim 1, wherein the average diameter of the cores is less than about 1 micrometer.
 15. The composition as defined in claim 1, wherein a mean value for the grain boundary layer is less than 50 nanometers.
 16. The composition as defined in claim 1, wherein an average distance from a grain boundary of one core to a grain boundary of an adjacent core in the sintered mass is less than about 1 micrometer.
 17. The composition as defined in claim 1, wherein the average thickness of the grain boundary layer is less than about 400 nanometer.
 18. The composition as defined in claim 1, wherein the sintered mass has a dielectric strength or breakdown field of greater than about 0.5 kV/mm.
 19. The composition as defined in claim 1, wherein the sintered mass has a non-linearity coefficient (α) of greater than about
 25. 20. The composition as defined in claim 1, wherein the sintered mass has a thermal profile comprising exposure to a sinter temperature of less than about 1050 degrees Celsius.
 21. A composition comprising a sintered reaction product of: transition metal oxide particles having an average diameter less than about 1 micrometer; sintering additive particles having an average diameter less than about 1 micrometer; and grain growth inhibitor additive particles having an average diameter less than about 1 micrometer, wherein the sintered reaction product has a thermal history that is less than about 1050 degrees Celsius.
 22. A composition, comprising: a sintered mass of particles comprising the reaction product of a transition metal oxide, a sintering additive, and a grain growth inhibitor additive; and, wherein the sintered mass has a density greater than about 98 percent of theoretical density for a composition comprising the transition metal oxide.
 23. A composition, comprising: sintered particles comprising a transition metal oxide, a sintering additive, and a grain growth inhibitor additive and defining grains; and the grains have grain boundaries that define the grains to have an average grain size of less than about 0.8 micrometers. 